Apparatus for measuring a winding temperature of electric machines

Abstract

An apparatus for measuring the winding temperature of electric machines uses an a.c. reference voltage source generating an a.c. reference voltage with a predetermined, non-line frequency, preferably below 10 Hz., a current detector detecting the current generated from the a.c. reference voltage and/or the a.c. reference voltage directly, and a measuring and evaluating device measuring the current to determine the winding temperature. The a.c. reference voltage is added geometrically to the line voltage and generates a current proportional to the winding conductance which is temperature dependent.

What is claimed is: 
     
       1. An apparatus for measuring a winding temperature of an electric machine fed by mains with at least one phase, comprising: a) an a.c. reference voltage source coupled in series with the mains, supplying the electric machine with an a.c. reference voltage having a predetermined, non-line frequency, causing a current having said predetermined frequency to flow through the winding of the machine, wherein said predetermined frequency is less than 10 Hertz and said a.c. reference voltage is between 1-2 percent of a line voltage of the electric machine, whereby any additional heating caused by said a.c. reference voltage is negligible relative to heating caused by the line voltage and to a permissible machine temperature; and   b) a current detector providing an output proportional to said current from which the conductance of the winding and the winding temperature can be determined.   
     
     
       2. The apparatus of claim 1 wherein the mains have a plurality of phases, further comprising a plurality of a.c. reference voltage sources supplying the machine with a plurality of a.c. reference voltages having a predetermined non-line frequency, whereby a plurality of currents having said predetermined non-line frequency flow through the winding of the machine. 
     
     
       3. The apparatus according to claim 1, wherein said a.c. reference voltage source comprises a function generator generating said a.c. reference voltaqe and a resonance transformer having a primary and a secondary, a voltage source coupled to the primary of said resonance transformer, and the secondary of said transformer coupled in series with the mains supplying said machine. 
     
     
       4. The apparatus according to claim 1, wherein said a.c. reference voltage source comprises a voltage source and a broad band transformer, said voltage source supplying said broadband transformer which provides said a.c. reference voltage to the machine. 
     
     
       5. The apparatus according to claim 1 wherein said machine is fed by a pulse-controlled a.c. converter and said pulse controlled a.c. converter comprises said a.c. reference voltage source modulated with an additional signal having said predetermined frequency. 
     
     
       6. The apparatus according to claim 1, wherein said a.c. reference voltage source comprises a d.c. voltage source with a switch for coupling said d.c. source in series with the mains and a pulse generator operating said switch at said predetermined frequency. 
     
     
       7. The apparatus according to claim 1, wherein said a.c. reference voltage source comprises a mains voltage modulator. 
     
     
       8. The apparatus according to claim 7, wherein said mains voltage modulator generates said a.c. reference voltage in at least three different predetermined frequency ranges which are evaluated separately. 
     
     
       9. The apparatus according to claim 1, wherein said current detector comprises a resonance transformer. 
     
     
       10. The apparatus according to claim 1, wherein said current detector comprises a broadband transformer. 
     
     
       11. The apparatus according to claim 1, wherein said current detector comprises a shunt. 
     
     
       12. The apparatus according to claim 1, further comprising a measuring and evaluating device having an input coupled to the output of said current detector. 
     
     
       13. The apparatus according to claim 12, wherein said measuring and evaluating device comprises a plurality of selective filters coupled to an input of an averaging unit calculating an average current. 
     
     
       14. The apparatus according to claim 12, wherein said measuring and evaluating device comprises a spectrum analyzer. 
     
     
       15. The apparatus according to claim 12, wherein said measuring and evaluating device comprises a synchronous power rectifier demodulating said current. 
     
     
       16. The apparatus according to claim 12, wherein said a.c. reference voltage source comprises a band spreading function generator modulating said a.c. reference voltage, and said measuring and evaluating device comprises a synchronous demodulator demodulating said current. 
     
     
       17. The apparatus according to claim 1, wherein said mains have a plurality of phases, a plurality of a.c. reference voltage sources supplying a.c. reference voltages to the mains and a plurality of current detectors, one for each phase and further including means for monitoring the symmetry of said currents in order to detect a short circuit in the winding or an exposed conductive part. 
     
     
       18. The apparatus according to claim 1, wherein said machine is an induction machine and said a.c. reference voltage source includes a further a.c. reference voltage source from which a speed signal can be derived and further including means to switch said further a.c. reference voltage source into said mains and means for measuring to detect the rotary speed of said machine having said further a.c. reference voltage and the output of said current detector as inputs.